The Ultraviolet Spectrum of DQ Herculis: Detection of Line and Continuum Pulsation

Details The Ultraviolet Spectrum of DQ Herculis: Detection of Line and Continuum Pulsation The Ultraviolet Spectrum of DQ Herculis: Detection of Line and Continuum Pulsation

May 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...462..428s&link_type=abstract

Astrophysical Journal v.462, p.428

Astronomy and Astrophysics

Astronomy

17

Stars: Novae, Cataclysmic Variables, Stars: Individual Constellation Name: Dq Herculis, Stars: Oscillations, Ultraviolet: Stars

Scientific paper

The old nova DQ Herculis was observed with the Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) in the 1150-2500 Å range. Spectra were taken with a time resolution of 4.08 s, allowing a search for pulsations of the UV light at the 71 s white dwarf spin period, analogous to those long known to be present in visible light. The integrated UV spectrum shows strong line emission in Lyα, N V, Si IV/O IV], C IV, and He II. The flux of the UV continuum, Si IV/O IV], C IV, and He II drop during the eclipse by the late-type star, indicating that most of this emission is generated near the white dwarf. There is a hump in the UV light curve prior to eclipse, due to a hot spot at the edge of the accretion disk with T ˜ 25,000 K and area ˜4 x 1018 cm-2.
Our FOS data yield the first report of coherent 71 s pulsed UV emission from DQ Her. Pulsations in the UV continuum are detected with half-amplitudes of 4.5% during the first HST orbit, 2.5% during the second HST orbit prior to eclipse, and 1.2% during the eclipse ingress. The UV amplitude is clearly time-variable, also known to be the case in visible light, as no continuum pulsations are seen during the third HST orbit (upper limit 2%). Most interesting, however, is the behavior of the UV emission lines. Prior to the eclipse ingress, Lyα is seen to pulse with at least a 16% amplitude, and displaced 0.27 in phase later than that of the continuum pulse. During the ingress, C IV pulses with an amplitude of 6%, displaced by 0.30 earlier in phase than the continuum. These phase shifts may indicate that the true period of the line emission is slightly different from the continuum pulsation. The UV pulsations clearly hold great promise as an aid in unraveling the complexities of this well-studied but still poorly understood system.
ROSAT has detected faint X-ray emission from DQ Her with LX(0.1-2.O keV) 1030 ergs s-1. While the observed X-ray emission is consistent with an origin from the late-type secondary star, the strength of He II λ1640 suggests there is an additional hidden X-ray component, of strength (LX 1034 ergs s-1) comparable to that seen in other DQ Her stars.

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